The invention relates to a mirror for the EUV spectral range, which guarantees monochromatisation of EUV radiation.
Optical systems in the EUV spectral range, which extends from about 5 nm to 40 nm, are based on multi-layer mirrors. They consist of alternating layer systems of two materials, which differ as much as possible in their optical constants. For some applications of EUV radiation, for example in the field of materials research, X-ray astronomy or the development of X-ray microscopes, monochromatisation of EUV radiation is necessary. For this, it is necessary to reduce the half width of the reflection of multi-layer mirrors.
Two approaches are known in the literature for this reduction of the half width:
It is known to carry out lateral structuring by reactive ion etching for a multi-layer mirror of the material combination molybdenum/silicon from R. Benbalagh, J. M. Andre, R. Barchewitz, M. F. Ravet, A. Raynal, F. Delmotte, F. Nridou, G. Julie, A. Bosseboeuf, R. Laval, P. Troussel, Nucl. Inst. Meth. Phy. Res. A 458(3) (2001), 650–655, and hence to achieve a reduction of the half width of the reflection of the multi-layer mirror by a factor 3 with respect to the full half width of the reflection of a molybdenum/silicon multi-layer mirror optimised for maximum reflectivity.
However, this process has the disadvantages that the lithographic structuring of the multi-layer is very expensive and also that for such a multi-layer mirror, a considerable reduction of the reflectivity to R=2% can be observed for an angle of incidence of about 45°—compared to this, a corresponding multi-layer mirror not lithographically structured in this manner has a reflectivity of R=40% for the same angle of incidence.
It is known to achieve a reduction of the half width of the reflection of molybdenum/silicon multi-layer mirrors by a reduction of the layer thickness of the molybdenum part layers from Y. C. Lim, T. Westerwalbesloh, A. Aschentrup, O. Wehmeyer, G. Haindl, U. Kleineberg, U. Heinzmann, Appl. Phys. A72 (2001), 121–124. However, in practice it is shown that a reduction of the molybdenum layer thickness is only possible to a limited extent, since it is only about 3 nm even for a standard multi-layer mirror.